1. Field of the Invention
The present invention relates to an acoustooptical modulator, such as those used in optical measuring devices.
2. Background Art
An acoustooptical modulator adds a modulation signal to a piezoelectric element, and causes periodic oscillation in the refractive index within an optical medium by ultrasonic waves produced therein, modulating the beam.
FIG. 4 is a block diagram showing an example of the arrangement of the conventional technology for an acoustooptical modulator.
In the arrangement shown in FIG. 4, the optical signal propagated by an optical fiber 51 is focused by a lens 52 and is incident on the acoustooptical modulator 58.
The ultrasonic band frequency signal produced by the oscillator 53 is incident on the piezoelectric vibrator 54 installed on the acoustooptical medium 58, and the ultrasonic waves produced by this piezoelectric vibrator cause periodic oscillation of the refractive index of the acoustooptical medium 58.
At this point, the optical signal incident from the optical fiber 51 is separated into the transmitted beam and the primary diffracted beam due to a diffraction grating being formed in the acoustooptical medium 58.
The optical signal output from this acoustooptical modulator transits a prism 55, and after being focused by a lens 56, is emitted by the optical fiber 57.
By switching the signal output by the above described oscillator 53 on and off, the diffracted photons within the acoustooptical medium are generated and extinguished. Due to this, in addition to use as an optical modulator, the arrangement shown in FIG. 4 realizes a mechanism for switching on and off an optical signal transiting between the optical fiber 51 and the optical fiber 57, and is widely used as, for example, a measuring device.
However, when using the acoustooptical modulator as a measuring device, loss due to the wavelength dependence of the acoustooptical modulator becomes a problem. FIG. 5 is a chart showing the wavelength-dependent characteristics of the loss of the acoustooptical modulator shown in FIG. 4.
As is apparent from FIG. 5, when the wavelength of the optical signal transiting within the acoustooptical modulator changes, ripple characteristics can be observed in the wavelength loss characteristics. This is believed to be due to polarization mode scattering produced by birefringence characteristics of the acoustooptical medium.
That is, when an optical signal having polarization mode scattering transiting the acoustooptical modulator enters the emitting optical fiber, inside optical fiber 57, interference due to the effects of mixing inside the optical fiber 57 is produced. In this case, the degree of interference changes depending on the wavelength of the optical signal.
Because of this, when the wavelength of the optical signal transiting inside the acoustooptical modulator changes, even if the power of the entering optical signal is constant, the power at the end of optical fiber 57 fluctuates. This is the ripple of the loss characteristics of the acoustooptical modulator.
The ripples having these loss characteristics become a significant problem when using this acoustooptical modulator as, for example, a measuring apparatus.